No sitting president has survived a serious primary challenge in the past 50 years. Here’s why Trump should be worried.

When a primary opponent arises and challenges a sitting president in their own party, the result is often the same: The incumbent is weakened and ultimately loses to their opponent in the general election.

While it is rare that an incumbent president faces a serious primary challenger, the few times it happened in the 20th century, it divided the party and drew out a process that was supposed to be a breeze.

Now, after a tumultuous two years in the White House, President Donald Trump is facing the prospect of a primary challenger, or perhaps several.

Former Massachusetts Gov. Bill Weld launched an exploratory committee to examine a run for president on the Republican ticket. Weld, who in 2016 was the vice-presidential nominee on the Libertarian Party ticket, has characterized Trump as unstable and unfit to be president.

“To compound matters, our president is simply too unstable to carry out the duties of the highest executive office — which include the specific duty to take care that the laws be faithfully executed — in a competent and professional matter,” Weld said at an event in New Hampshire upon announcing his exploratory committee. “He is simply in the wrong place.”

Several other Republicans are mulling challenges against Trump from a centrist position, including Maryland Gov. Larry Hogan and former Ohio Gov. John Kasich, who unsuccessfully ran for the GOP nomination in 2016.

Primary challenges against sitting presidents typically weaken them. There are several examples of Republicans and Democrats mounting insurgent bids that ultimately resulted in their party’s president failing to secure another term — even if they do not even come close to winning the nomination.

An examination of the 1992 Republican presidential primary is a somewhat of a reverse of the situation that Trump could soon find himself in.

Patrick Buchanan, a former senior aide to both President Richard Nixon and President Ronald Reagan, mounted a primary challenge against Bush. Buchanan ran from nationalist, right-wing approach. He was much further to the right than Bush, promoting a nationalist, populist agenda against what many on the right viewed as a weak-kneed globalist.

“But by espousing positions on trade, immigration and the role of the United States in the world, views tinged with a kind of isolationism and protectionism that are anathema to many free-market and globalist conservatives, Mr. Buchanan is seen by some as an imperfect spear with which to impale the President,” The New York Times’ Steven Holmes wrote in 1992.

Buchanan’s challenge rattled the Bush campaign. While Buchanan never defeated Bush in any of the primary races, he produced strong results and, in some cases, exposed ideological rifts within the Republican Party, including in the early primary state of New Hampshire.

Bush would go on to lose his reelection fight against Bill Clinton.

Kennedy vs. Carter

When Massachusetts Sen. Ted Kennedy challenged Jimmy Carter in 1980, the US economy was not in top shape, prompting questions about Carter’s strength and ability to win reelection.

Carter and Kennedy were at odds for quite some time. But the primary race brought that bad blood to the forefront of American politics.

Kennedy would go on to severely damage Carter in the primary, winning 12 states compared with Carter’s 29.

Kennedy, who remained in the Senate and died in 2009, eventually acknowledged he would not win the primary and bowed out of the race. But Carter would go on to lose to Ronald Reagan, who was the California governor at the time.

Before Reagan defeated Carter in 1980, he mounted a primary challenge against President Gerald Ford, who had recently taken over as the commander in chief in the wake of the Watergate scandal and the resignation of Richard Nixon.

Ford, who was appointed vice president after the resignation of Spiro Agnew, became president in the most unconventional way. Less than one year before becoming president, Ford was the House minority leader.

The scandal-ridden administration that collapsed as a result of Watergate opened the door for an insurgent primary challenge. Reagan gave Ford a run for his money during the primary but fell short of the number of delegates needed to win the nomination outright.

Then at the 1976 Republican National Convention in Kansas City, Missouri, Reagan contested Ford’s nomination. Ford won the nomination, but delegates replaced Vice President Nelson Rockefeller’s nomination with Kansas Sen. Bob Dole.

Ford ultimately lost to Carter in the general election.

Other primary challenges created problems for incumbents

A handful of other instances of primary challengers attempting to oust incumbent presidents have shaken up races but have done far less damage.

When Sen. Eugene McCarthy of Minnesota challenged President Lyndon B. Johnson in 1968 at the height of the Vietnam War, the Democratic Party devolved into chaos, ultimately leading to the complete restructuring of the primary system for the following presidential election.

New York Sen. Robert Kennedy then challenged Johnson for the Democratic nomination, too. What followed became one of the most chaotic primary campaigns in US history.

Johnson bowed out of the race and was replaced at the top of the ticket by Vice President Hubert Humphrey.

The night he secured victory in the California Democratic primary, Kennedy was assassinated by Sirhan Sirhan, a Palestinian-Jordanian national angered by Kennedy’s support for Israel.

Humphrey would go on to become the nominee at the Democratic National Convention and lose to Richard Nixon.

Whether Trump will face the same fate as so many other incumbent presidents who faced primary challengers is still unclear. But recent polling shows there is indeed an appetite for someone who is not Trump within the Republican Party, albeit a still relatively small one.